Most industrial waters contain alkaline earth metal cations such as calcium, barium, magnesium, etc., and several anions such as bicarbonate, carbonate, sulfate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until these product solubility concentrations are no longer exceeded. For example, when the ionic product of calcium and carbonate exceeds the solubility of calcium carbonate, a solid phase of calcium carbonate will form.
Solubility product concentrations are exceeded for various reasons such as partial evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on surfaces of the water-carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. This scale is an expensive problem in many industrial water systems, such as recirculating cooling water systems in cooling towers, in that the scale causes delays and shutdowns for cleaning and removal of the scale.
Desalination of saline or brakish water can be accomplished by means of reverse osmosis. In its simplest form, osmosis uses a membrane which is semi-permeable to water but which rejects certain dissolved salts. If pure water is separated by a semi-permeable membrane from a salt solution, pure water will flow through the membrane to the salt solution side until osmotic equililbrium is reached. However, if positive pressure is applied to the salt solution to overcome the osmotic pressure, the flow will be reversed and water will flow from the salt solution through the membrane to the pure water side. This is what is meant by reverse osmosis.
In a reverse osmosis water purification system, impure water is pumped under high pressure into the system where it contacts the membrane. The product water is 95-99% free of dissolved minerals.
The continued efficiency of a reverse osmosis system depends on the maintenance of the membrane in an unfouled condition. A critical problem experienced by this system is fouling of the membrane by precipitation of scale. It is, therefore, desirable to prevent scale build-up or to prolong the time between membrane changes by reducing scale precipitation in the water stream that is passed through a reverse osmosis system.
Primary scale inhibiting compounds have been added in the past in treatment of water to inhibit precipitation of scales, particularly calcium carbonate. Examples of such primary scale inhibiting compounds include amino phosphonic acids and phosphonates, diphosphonic acids, phosphonoalkane tricarboxylic acids, polyphosphoric acids, polyol phosphate esters, maleic anhydride copolymers, acrylic polymers, and others. Although the above primary scale inhibiting compounds may be excellent scale inhibitors for calcium carbonate scale, they can form insoluble precipitates under conditions encountered in water treatment, particularly cooling water and reverse osmosis systems. The insoluble precipitates that are formed are calcium salts of scale inhibitors, i.e. Ca-phosphonates, which are frequently formed at normal use--concentrations of primary scale inhibitors.